This invention relates to wire drawing dies of the type comprising a wear-resistant insert, such as a diamond insert or body, mounted in a suitable support.
Monocrystalline diamond wire drawing die inserts are extensively used in the industry. One of the drawbacks of monocrystalline diamond inserts for wire drawing dies is the fact that the inserts wear in a non-uniform pattern following crystallographic directions of lower wear resistance. As a consequence the cross section of the wire being drawn may change with time as the insert wears.
Further, monocrystalline diamond is intrinsically expensive to produce in large quantities.
A further problem with synthetically produced monocrystalline diamond relates either to metallic inclusions or a metallic phase present in the diamond crystals due to the synthesis process. This metallic component may lead to thermal instability resulting in premature wear or cracking.
Also presently used in the industry are wire drawing die inserts made of a polycrystalline diamond. This material does not present the preferential crystallographic wear pattern of crystalline inserts. However, it contains a metal binding face in its matrix such as cobalt which introduces an element of thermal instability in this material. Due to the presence of this matrix the wear is not always sufficiently smooth.
To avoid these and other problems introduced by the metal phase in polycrystalline diamond (PCD), some workers have resorted to leaching out the metal phase by chemical etching. Even this procedure presents disadvantages, however. First, it is difficult to achieve anything approximating complete and efficient removal of the metal phase. Second, the leaching removes the metal, leaving voids and results in a porous PCD material, the strength of which decreases with increasing porosity. Third, a porous structure is more difficult to polish to a smooth surface finish than a dense polycrystalline body.
Processes are known whereby diamond is synthesised in the gas phase. These methods are known as chemical vapour deposition (CVD) and the diamond produced by such processes is known as CVD diamond. These processes generally involve providing a mixture of hydrogen gas and a suitable gaseous carbon compound such as a hydrocarbon, applying sufficient energy to the gas to dissociate the hydrogen into atomic hydrogen and the gas into active carbon ions, atoms or CH radicals and allowing such active species to deposit on a substrate to form diamond. Dissociation of the gases can take place by a variety of methods such as hot filament, plasma assisted methods or plasma jet.